1. Field of the Invention
The present invention relates to an image correction method and an image correction apparatus for correcting lightness and color of a region such as human face and skin in a digital image. The present invention also relates to a program therefor.
2. Description of the Related Art
In some digital photograph images obtained by photography with a digital camera or by reading a photograph output on a paper medium or the like with a scanner, lightness and color thereof are not represented in an appropriate or desired manner due to a photography condition and a characteristic of a subject. Although correction of lightness and color may be necessary, application of predetermined correction processing on an entire image is not appropriate in some cases.
For example, in the case of a photograph image whose subject is a person photographed in a state of underexposure or in presence of backlight, lightness of the face of the person may need to be corrected since the face is dark. However, if gradation correction processing causing lightness of the face to become appropriate is applied to the entire image, white compression may occur in the background. Furthermore, in the case where only a person at an end of a rear row looks dark due to insufficient illumination thereon in a class photo photographed in a photographic studio or the like, or in the case where one of two people in a photograph image is extremely tanned and the face of the person thus looks dark, it may be preferable for lightness of the dark face of the person to be corrected according to lightness of other faces or the other face. However, if gradation correction processing is applied to the entire image for causing only the face of the person to become lighter, the faces of others or the face of the other person become(s) too light beyond an appropriate range.
In order to avoid such a problem, partial correction such as correction of only the face or skin of a person may be desired, instead of correction of an entire image. For partial image correction, various kinds of methods have been proposed.
For example, an identification photograph system has been proposed in U.S. Patent Application Publication No. 20010005222 for carrying out image processing only on a person. In this system, a region of a person is automatically separated from a region of background in an image, and a skin color of the person is automatically adjusted while the background is replaced with a background prepared in advance. The person after the automatic adjustment is then composited with the changed background.
An automatic dodging technique for face has also been described in Japanese Unexamined Patent Publication No. 10(1998)-126184. In a method described therein, a face region is extracted, and a skin region therein is subjected to preferable processing. The face region after the processing is then composited with the remaining region.
In Japanese Unexamined Patent Publication No. 2001-209802, a method of face extraction has been described. In this method, a face template comprising a wire frame or the like is displayed beside a photograph image, since a face may be extracted erroneously if extracted with reference to only a color of a pixel in a skin-color region in a specified region. Therefore, a position of a face in the image is matched with a position of the face template, and the face is extracted based on a result of the matching.
Furthermore, a method has been proposed in U.S. Pat. No. 7,024,035 for improving efficiency of automatic red-eye correction through easy specification of a red-eye region by an operator in advance, although this method is not related directly to face region extraction or correction on face. In this method is disclosed how a specific region in a partial region is extracted after specification of the partial region.
However, each of the 4 methods described above respectively have problems described below.
The identification photograph system described in U.S. Patent Application Publication No. 20010005222 deals with an identification photograph with a simple background. Therefore, automatic separation of a person from the background is comparatively easy, and hair or clothes can also be extracted. However, since a photograph generally has a complex background, this system cannot be applied simply to a general photograph. In addition, automatic adjustment of skin color may not result in appropriate correction, and means for manual correction is necessary.
In the automatic dodging method for face described in Japanese Unexamined Patent Publication No. 10(1998)-126184, a face having been subjected to correction is simply composited with the background. Therefore, depending on accuracy of face region extraction, an outline of face may be distorted, and an unnatural image may result from the compositing. In addition, a combination of hair color with face may lead to generation of an unnatural image, since only a skin region in face is considered in dodging. Furthermore, since a photograph generally has a complex background, probability of failure of the automatic face detection is high.
In the face extraction method described in Japanese Unexamined Patent Publication No. 2001-209802, an operation for positioning a face template is troublesome and inefficient.
The region specification method described in U.S. Pat. No. 7,024,035 only discloses the method of specific region extraction from a manually specified region, and does not propose an efficient method of face or skin correction.
In addition to the methods described above, manual partial image processing may be carried out with use of image processing software such as Photoshop® available on the market. However, operations therefor are complex and inefficient.